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Nature. 2014 Nov 6;515(7525):88-91. doi: 10.1038/nature13836. Epub 2014 Oct 19.

Suppression of cooling by strong magnetic fields in white dwarf stars.

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Special Astrophysical Observatory of Russian Academy of Science, Nizhnij Arkhyz, Zelenchukskiy region, Karachai-Cherkessian Republic 369167, Russia.
Institute for Astrophysics, Georg-August-University, Friedrich-Hund-Platz 1, D-37077, Göttingen, Germany.
Department of Physics, Royal Military College of Canada, PO Box 17000 Station Forces, Kingston, Ontario, K7K 7B4, Canada.
Crimean Astrophysical Observatory, Nauchny, Crimea 98409, Ukraine.
Instituto de Astronomía, Observatorio Astronómico Nacional San Pedro Martir (SPM), Universidad Nacional Autónoma de México, 22860 Ensenada, Baja California, Mexico.
1] Instituto de Astronomía, Universidad Catolica del Norte, Avenida Angamos 0610, Antofagasta, Chile [2] Pulkovo Observatory, Pulkovskoe Shosse 65/1, Saint-Petersburg 196140, Russia.
Armagh Observatory, College Hill, Armagh BT61 9DG, UK.
Korea Astronomy and Space Science Institute 776, Daedeokdae-ro, Yuseong-gu, Daejeon 305-348, South Korea.
1] Instituto de Astrofísica de Canarias, 38200, La Laguna, Tenerife, Spain [2] Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain.
Instituto Nacional de Astrofísica, Óptica y Electróníca, Apartado Postal 51 y 216, 72000 Tonantzintla, Pueblo, Mexico.
European Southern Observatory, Alonso de Córdova 3107, Santiago, Chile.


Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.


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